This invention pertains to the preparation of allylic cycloalkenols and more particularly to the autoxidation of the corresponding cycloolefins with elemental oxygen and catalytic amounts of soluble cobalt and copper compounds together with a catalyst activating amount of a carboxylic acid.
Allylic cycloalkenols such as 2-cyclopentenol and 2-cyclohexenol are useful intermediates for the synthesis of polyfunctional epoxides for epoxy resin systems. 2-Cyclopentenol is of particular interest since it can be etherified and then epoxidized to afford bis-(2,3-epoxycyclopentyl) ether, a diepoxide that reacts with aromatic diamines to provide condensation resins having excellent high-temperature properties. This series of reactions is delineated below. ##STR1##
It is therefore an object of this invention to provide a facile means of producing allylic cycloalkenols.
It is a further object to produce allylic cycloalkenols by a rapid oxidation of cycloalkenes to their corresponding allylic cycloalkenols and/or allylic cycloalkenol esters with a minimum of formation of by-products, such as, ketones and high boiling resinous materials.
It is still a further object to effect the production of allylic cycloalkenols at a high efficiency.
It is known that the autoxidation of hydrocarbons proceeds by a free-radical chain reaction to form hydroperoxides as the primary product. When certain soluble metal salts are present, they can react with the hydroperoxides to give free radicals, thereby starting new reaction chains and speeding up the reaction. This is illustrated below for the oxidation of cyclopentene in the presence of cobalt salts. ##STR2## Cobalt salts are particularly strong catalysts for autoxidation and hydrocarbons. Other metal salts, such as those of copper, will react with hydroperoxides in only one way and therefore do not behave as true catalysts since they cease to provide a source of radicals when all of the metal ions have been converted to the "inactive" valence state. In the case of copper it is the cuprous ion which can furnish radicals by the reaction: EQU Cu.sup.+ +ROOH.fwdarw.Cu.sup.++ +OH.sup.- +RO.
It is known that cupric ions are good oxidizing agents for converting free radicals to carbocations. In the case of cyclopentenyl radical a reaction would be: ##STR3## The usefulness of cobalt salts for catalyzing the autoxidation of cyclohexene was described in U.S. Pat. No. 2,223,500. It was stated herein that the principal product was 2-cyclopentenone, the principal by-product which is to be avoided in the instant invention.
The activating effect of carboxylic acids on the cobalt-catalyzed autoxidation of hydrocarbons has also been described. (E. H. Farmer et al., J.C.S., 1942, 121-30).
In British Pat. No. 635,054, the use of a combination of cobalt and copper acetates for the oxidation of acetaldehyde to acetic acid and acetic anhydride was described. This combination was also used by Union Carbide Corp. in the oxidation of acetaldehyde to these products.
The use of chain-transfer agents to improve the efficiency of 2-cycloalkenol and its ester has not been described in the prior art though there are many examples of toluene, acetone, and other solvents with chain-transfer properties being used as solvents to the autoxidation of other types of substrates. In such examples, the solvents were used for their solvent properties and not for their chain-transfer properties.
Allylic oxidation of olefins is a common phenomenon and has often been conducted in either the liquid or vapor phase in the presence of various types of catalysts or initiators or even, sometimes in their absence. Cyclopentene is particularly susceptible to oxidation by a free radical mechanism. Criegee et al. has shown in Ber. 72, 1799-1804 (1939) that liquid-phase autoxidation at 20.degree. C. initiated by ultraviolet light gave 2-cyclopentenyl hydroperoxide as the initial product. Van Sickle et al., J.A.C.S. 87, 4824-32 and 4832-37 (1965) obtained similar results using 2,2'-azobisisobutyronitrile (ABN) and 2-cyclopentenyl hydroperoxide itself as initiators.
Synder et al. J.A.C.S. 81, 4299-4300 (1959) claimed that the light-catalyzed liquid-phase autoxidation of cyclopentene in the presence of acetic anhydride gave cyclopentenone and 2-acetoxy-3,4-dihydro-[2H]-pyran.
The use of transition metal compounds as catalysts for the liquid-phase oxidation of hydrocarbons has been the subject of several studies.
There is a dearth of information on the metal-catalyzed autoxidation of cyclopentene in the literature. Of that available the following is representative:
Collman et al., J.A.C.S. 89, 4809-11 (1967), conducted the autoxidation of cyclopentene using iridium and rhodium complexes as catalysts to produce cyclopentenone and cyclopentene oxide.
East German Pat. No. 81650 issued on May 5, 1971 discloses the use of Rh(P.phi..sub.3).sub.3 Cl and a large amount of t-butyl hydroperoxide (with oxygen) for the oxidation of cyclopentene at 45.degree. C. This reaction was slow giving 2-cyclopentenol in 7% yield along with 10% of 2-cyclopentenone.
Several liquid-phase autoxidations of cyclohexene have been described in the literature. Gould et al., Journal of Catalysis 13, 238-44 (1969) showed that the principal volatile products were cyclohexene oxide, 2-cyclohexenone, 2-cyclohexenol, and 2-cyclohexenyl hydroperoxide.